Steel City 54171N-25R 4IN 2-1/8 OCTO BX 1/2KO NM CBLE 25

The Steel City 54171N octagon box has a 22.5 cubic inch volume, which is what you use for NEC box fill math. At 2.00 cubic inches per 14 AWG conductor that works out to 11 conductors, and at 2.25 per 12 AWG it is 10, before you subtract for the device yoke, clamps, and grounds. The internal cable clamps and any fixture stud count against that total, so a typical ceiling fixture box with two 12/2 NM cables, grounds, and clamps leaves you plenty of room.

Yes. At 4 inches diameter by 2-1/8 inches deep, the box sits well within the 3-1/2 inch depth of a 2x4 stud or a standard joist, so you have room behind it for the cable to turn. It is a new-work box for mounting into the wall or ceiling framing before the drywall goes on, not a retrofit box for cutting into finished surfaces.

Yes, it is built for nonmetallic sheathed cable with integral NM cable clamps, so you do not need to add separate connectors for Romex runs. Note that the clamps occupy volume in the box, which counts as one conductor deduction in your 22.5 cubic inch fill calculation.

The 54171N is a 1/2 inch knockout box, so it takes standard 1/2 inch trade size connectors and fittings. If you are transitioning to EMT or flex at the box, use a 1/2 inch connector rather than trying to enlarge the opening.

A 4 inch octagon like the 54171N is the usual choice for ceiling luminaires and for junction points, because standard fixture canopies and octagon covers match the 4 inch round pattern. A 4 inch square box gives more volume and accepts device rings for switches and receptacles, so it is the better pick when you are mounting a device or pulling more conductors than the 22.5 cubic inch octagon allows. For paddle fans you need a box specifically listed for fan support, not a standard outlet box.

The selling unit is each, so one unit gets you one box even though the part designation references a 25 count carton. Order the quantity you need by piece.

The box itself is just the enclosure. Plan on a 4 inch octagon or round cover plate (or the fixture's own canopy) to close it, plus a fixture stud or crossbar if the luminaire mounts on 3-1/2 inch centers. Cable clamps are already built in for NM, so Romex connectors are not required, but any conduit entry into the 1/2 inch knockouts needs its own connector.

Drawn steel, silver finish, manufactured in the United States. Drawn construction means the body is formed from a single piece rather than welded from sheet, so there are no side seams. Steel is what you need where metal raceway is used or where the jurisdiction or spec calls for metallic boxes rather than PVC.